This is based on a flawed reading of the GPL. The GPL covers the distribution of program code. The license specifically states that “The act of running the Program is not restricted”. (Furthermore: “Activities other than copying, distribution and modification are not covered by this License; they are outside its scope.”)
The terms you are all referring to relate to the distribution of the software, not the running of the software. Wikipedia.org, does not distribute the software, that is MediaWiki.org's job. If Wikipedia wanted to, we could remove all licensing information from the software and it would still be completely legal. The GPL *only* comes into effect once you start distributing the software.
This is why other licenses such as the Affero General Public License have been written, to stop people using and modifying software like Mediawiki, but failing to release their modifications back to the community.
The current method of distributing Mediawiki via Mediawiki.org is perfectly complaint with the GPL.
-- Chris